Thread: How to measure cartridge heradspace length and assess your resizing.

Sometimes, reloaded ammo is "hard to chamber".
Usually, this is because the case hasn't been sized down enough in the die.
Usually this is a full length (FL) sizing die.
Sometimes people will just size the neck to reduce diameter enough to hold a projectile (neck die). After a few firings the shoulder also has to be pushed back (shoulder bump die).
Sometimes sizing is ineffectual due to the press stretching a little, and the die needs to be tightened up so it keeps travelling a short way after it contacts (typically a quarter turn of the die mounting screw). This is sometimes described as "cam over"
You need a way to measure how much you are sizing them.

The headspace dimension of a case is measured from the base to a standard diameter part way down the shoulder.

These standard dimensions are specified on the american SAAMI web site.
Chamber dimensions are shown on the european CIP web site but not the cartridge headspace dimension.


SAAMI specifications
https://saami.org/wp-content/uploads...sting-Copy.pdf

CIP
https://bobp.cip-bobp.org/en/tdcc_pu...idge_type_id=1

To measure your own case before and after sizing, you will need :
calipers
A headspace guage. This is an insert ring of specified diameter hole which fits over the shoulder of the case (silvery aluminium in this pic). There is a body part that fits onto your caliper and the insert fits into that. (red part on the left in this pic) They come in sets with several sizes to measure a range of cartridges. You can use the same body with smaller inserts that fit onto your bullets to measure seating depth too.
It's nice to have a larger flat surface to rest the case base on rather than balancing it straight on the caliper (the flat red "anvil" on the right in the picture below).



For example, here is the SAAMI spec sheet for the 270WSM



So, you can see that you will need an insert of 0.4450" internal diameter and the length needs to be 1.723 to 1.730" after sizing.
That's quite a wide range.
Each chamber is cut a bit differently.
You can measure your fired cases to estimate the size of your chamber and then try to resize your cases to about 0.003" shorter.
They will feed and the bolt should close easily. By minimising the amount of sizing you do, you will make your cases last longer.

Thanks Bagheera, good information.

Originally Posted by Bagheera

Each chamber is cut a bit differently.

At lease twice I've ground the base (off) of FL dies as a means of achieving easy chambering on the sized cases. I didn't have the appropriate gauges so couldn't confirm where the problem lay and whether the chambers were still within specification, but see now from your post the comment on the allowable range. Anyway, a hard cam-over was still not enough on either. They wouldn't have been factory-fitted barrels though. The most recent was a re-chambered M70. Avoiding the additional rework required on a M70 to fix up having accidentally taken the reamer in a tad far is the likely explanation.